The reading room (Bates Hall) at the main branch of the Boston Public Library.
This and the Schwarzman Building branch of the New York Public Library are two of my favorite buildings. Not just libraries–I mean just buildings to wander around and look at. And get lost in.
Specs: Canon EOS 7D with EF-S 18-55mm f/3.5-5.6 IS @ 18mm, 1/320s @ f/5. Post-processed in Adobe Lightroom 5.